Blood Urea Nitrogen (BUN)
Also called: BUN, Blood urea nitrogen, Urea
BUN is a kidney-cleared waste product from protein metabolism. Interpreted alongside creatinine; the BUN/creatinine ratio distinguishes pre-renal causes from intrinsic kidney disease.
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What does Blood Urea Nitrogen (BUN) measure?
Urea is the liver's way of disposing of nitrogen from broken-down proteins. The kidneys filter and excrete it. BUN rises when the kidneys filter less (kidney failure) or when there's more urea to clear (high protein intake, GI bleed, dehydration). The BUN/creatinine ratio adds diagnostic value: a ratio >20 with rising BUN points at dehydration or upper-GI bleed; a ratio around 10–15 suggests true intrinsic kidney disease where both markers rise proportionally.
What does a high Blood Urea Nitrogen (BUN) level mean?
- Dehydration — most common cause; ratio with creatinine is high.
- Chronic kidney disease — usually with elevated creatinine and reduced eGFR.
- Acute kidney injury — rapid rise of both BUN and creatinine.
- Upper gastrointestinal bleed — blood acts as a protein meal, raising BUN.
- High protein intake, corticosteroids — modest rises.
- Congestive heart failure — reduced renal perfusion.
What does a low Blood Urea Nitrogen (BUN) level mean?
- Severe liver disease — reduced urea synthesis.
- Low protein intake.
- Pregnancy — increased renal clearance.
When to discuss with a doctor
BUN elevation should always be interpreted with creatinine and eGFR. Mediora.AI computes the BUN/creatinine ratio when both values are present and uses it to flag the dehydration-vs-kidney-disease pattern.